The present invention relates to a means and method of attaching an ultrasonic transducer to a surface. It is frequently necessary to bond ultrasonic transducers to surfaces in an environment in which the bonding material will be subject to high temperatures and radiation. One example is the use of ultrasonic flowmeters to monitor flow in the primary loop of nuclear power generating systems. The bonding material in these situations may be subjected to temperatures as high as 650.degree. F. and radiation levels as high as 50R (Roentgen) per hour. Investigations have shown that conventional, commercially available ultrasonic couplants are unsuitable for such conditions. For example, the commonly available colloidal grease type couplants will exhibit an excessive degree of thermal outgassing with eventual loss of physical properties and loss of mechanical bonding. Such changes would initially lead to constantly changing ultrasonic modulation which could generate erroneous ultrasonic data. Based on outgassing data, the in-service life of common high temperature couplants would be no greater than 700 hours at the temperature encountered in nuclear reactor service. It is further estimated that under irradiation conditions, and with subsequent loss of adhesion and mechanical properties due to both the thermal and irradiation embrittlement, the useful service life of commercial couplants would be no greater than 50 hours. Because of this relatively short predicted service life, frequent changing of the couplant would be necessary. Radiation exposure levels as well as inaccessibility of the monitoring sites would make such a practice prohibitive.
Accordingly, there has been a longstanding need for a couplant which:
(1) Provides required acoustical properties in the form of a sound path of good acoustical impedance between a transducer and the supporting wave-transmitting substrate, pipe or reactor flow passage to which it is attached;
(2) Withstands thermal cycling from room temperatures to temperatures over 450.degree. F., which are common in nuclear power generating system flow path structure surfaces upon which the flowmeter may be mounted;
(3) Maintains stable acoustical properties at temperatures over 450.degree. F., and typically in the range of from 500.degree. F. to 650.degree. F., for periods of time which may be as great as multiyear periods between planned nuclear reactor outages;
(4) Provides a minimum of irritating fumes or outgasses which may disturb the sound path;
(5) Is resilient and of a viscosity which provides for ease of application and use; and,
(6) Does not require expensive surface preparation of the substrate or nuclear power generation system flow path structure surfaces in the field, such as by grinding or machining.
In search of a couplant which would answer all of these longstanding needs, the laboratories of the assignee of the present invention produced the invention of U.S. Pat. No. 3,970,504 and subsequently began the study of eighteen candidate high temperature and lubricant release materials which were available commercially for other uses. Eleven of the candidates were immediately dismissed as chemically unsuitable in that they turned out to have objectionably high sulfur or lead contents. Two of the remaining seven had an inability to perform the ultrasonic couplant function above 350.degree. F. Another candidate had a viscosity which was too stiff for easy use as an ultrasonic couplant. Four were worthy of further testing with regard to hazardous or irritating fumes and controlled use on actual weldments. They were rated and the most likely three to solve the problems came from the same source, Dow Corning Corporation of Midland, Mich., 48640. A review of materials from that source, indicated the availability of a grease-like silicone fluid material heavily filled with heat conductive metal oxides and designated "DOW CORNING 340 Heat Sink Compound".
After laboratory oven test for high temperature longevity of the heat sink compound as a potential ultrasonic couplant were successfully performed, the material was used at two nuclear reactor sites on pipe surfaces carefully prepared by grinding the flowmeter transducer attachment area. Although the material withstood the environment, the surface preparation was found to be too expensive to be a practical field procedure for use. Thus, use of the material as a successful ultrasonic couplant for withstanding the high temperature conditions, without any special grinding or machining surface preparations had to be established. Tests to accomplish this were then successfully carried out in a laboratory autoclave. Testing at a nuclear power generation site ultimately was performed which confirmed these laboratory results.
The "DOW CORNING 340 Heat Sink Compound" used is known for application to the base and mounting studs of transistors and diodes and silicon controlled rectifiers. It is promoted as an effective thermal coupler for any heat sink device where efficient cooling is required. It is also mentioned in the product information literature as a high voltage corona-suppressant, nonflammable coating in connections for flyback transformers in TV sets and similar applications. Although it is stated that it will not dry out, harden or melt, even after long term exposure to temperature up to 200.degree. C. (392.degree. F.), nothing suggests it might be an ultrasonic couplant in a high temperature (over 450.degree. F.) environment.